1. Field of the Invention
The invention relates to sol-gel processing methods.
2. Discussion of the Related Art
The manufacture of optical fiber, in one approach, involves the fabrication of an overcladding for an optical fiber preform that surrounds an inner cladding and core. The overcladding, unlike the core and inner cladding of a fiber preform, does not have to meet precise specifications. Thus, efforts to speed manufacture of preforms have often focused on easier and faster methods of forming the overcladding. One manner of forming the overcladding is the use of a sol-gel process. However, sol-gel methods tend to encounter cracking during the overcladding tube formation and subsequent drying process. Methods that suppress such tendency include, for example, the use of supercritical drying and/or the use of drying control chemical additives (DCCA), both of which are relatively expensive and laborious. Other sol-gel processes have involved the precipitation of silica particles from solution. However, such precipitation processes typically involve the use of alkali silicates, and thus require further processing steps to remove the alkali metal ions.
U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference, discloses a sol-gel process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In this process, a colloidal silica dispersion, e.g., fumed silica, is obtained having a pH of 2 to 3. To obtain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 11 to about 14 by use of a base. Typically, a commercially-obtained dispersion is pre-stabilized at such a pH value by addition of a base such as tetramethyl ammonium hydroxide (TMAH). Upon introduction of the TMAH, substantially complete dissociation to TMA.sup.+ and OH.sup.- occurs, raising the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica, it is believed, takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction: ##STR1## The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. In this state, the zeta potential of the particles is at a negative value. (Zeta potential is the potential across the diffuse layer of ions surrounding a charged colloidal particle, and is typically measured from electrophoretic mobilities--the rate at which colloidal particles travel between charged electrodes placed in a solution.)
At a later stage in the process, as discussed in Col. 15, lines 39-65 of U.S. Pat. No. 5,240,488, a gelling agent such as methyl formate is added to reduce the pH. It is possible to use other esters, as well. The ester reacts with the water and/or base to generate H.sup.+ ions. The negative character of the silica particles is neutralized according to the following reaction: ##STR2## A sufficient amount of the ester must be introduced to provide enough H.sup.+ ions to neutralize the silica to a degree where gellation is induced. (Gellation, as used herein, indicates that the colloidal silica particles have formed a three-dimensional network with some interstitial liquid, such that the dispersion becomes essentially non-flowing, e.g., exhibiting solid-like behavior, at room temperature.)
As mentioned above, a quaternary ammonium hydroxide commonly used for stabilizing silica sols is TMAH. A significant problem with TMAH is that during heating of the gelled tube formed from the dispersion, the TMAH breaks down into trimethyl amine, which has an extremely foul odor. Related compounds, e.g., TMAH's ethyl homolog, have similar odor problems. Similarly, methyl formate produces a by-product of formaldehyde, which is also an odorous material. Substitute materials which do not have such characteristics would be advantageous, as would sol-gel methods that require less TMAH and/or methyl formate.